Wide angle optical system and image pickup apparatus using the same

ABSTRACT

An optical system comprises, in order from the object side, an aperture stop, a first lens with positive refracting power, a second lens with negative refracting power, and a third lens, both surfaces of the third lens are an aspherical surface in which refracting power varies in accordance with distance from the optical axis in such a way that the both surfaces have a convex shape facing toward the object side in the vicinity of the optical axis and have a concave shape facing toward the object side in the vicinity of the outer circumference, and the following conditions (1) and (2) are satisfied:
 
2.2&lt; d   2   /d   3 &lt;7  (1)
 
−0.04&lt; f/f   3 &lt;0.04  (2)
 
where d 2  is the thickness of the first lens on the optical axis, d 3  is a space distance between the first and second lenses on the optical axis, f is a focal length of the whole of the wide angle optical system, and f 3  is a focal length of the third lens.

This application claims benefits of Japanese Patent Application No.2008-288141 filed in Japan on Nov. 10, 2008, the contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a wide angle optical system and an imagepickup apparatus using the same. Especially, this invention relates to asmall wide angle optical system which is favorable for photographing anobject which is located at a relatively close position and relates to animage pickup apparatus using the same.

2. Description of the Related Art

As cellular phones, mobile terminals, and notebook computers have becomethinner recently, the thin apparatuses require a camera module which isthinned in the direction of the length along the optical axis of theoptical system to the utmost limit.

In order to meet the requirement, a large number of single-focus opticalsystems which are composed of about two or three aspherical lenses havebeen proposed for the purpose of using the single-focus optical systemsfor a camera module.

There is also a proposal to use for a camera module an image pickupelement which has been developed in recent years and the sensitivity ofwhich is not deteriorated by light which is slantingly incident on theperiphery of the image pickup element. The use of such image pickupelement for a camera module makes it possible to bring an exit pupilposition of an optical system near to the image pickup element, so thatit is possible to shorten the total length of the optical system by theuse of the image pickup element. As a result, it is possible to thin thecamera module. This is the reason why the use of such image pickupelement for a camera module has been proposed.

Japanese Patent Kokai No. 2007-3768, Japanese Patent Kokai No.2007-47513, and Japanese Patent Kokai No. 2007-58153 disclose an opticalsystem which is composed of three lenses and in which an exit pupilposition is nearer to an image pickup element, as one example of suchoptical systems. The optical system which is disclosed in the patentliteratures is made as a telephoto type optical system in which apositive lens, a negative lens, and a negative lens are arranged inorder from the object side of the optical system, with the aim ofshortening the total length of the optical system more.

SUMMARY OF THE INVENTION

A wide angle optical system according to the present invention ischaracterized in that: the wide angle optical system comprises, in orderfrom the object side, an aperture stop, a first lens with positiverefracting power, a second lens with negative refracting power, and athird lens; both surfaces of the third lens are an aspherical surface inwhich refracting power varies in accordance with distance from theoptical axis in such a way that the both surfaces have a convex shapefacing toward the object side in the vicinity of the optical axis andhave a concave shape facing toward the object side in the vicinity ofthe outer circumference of the lens; and the following conditions (1)and (2) are satisfied:2.2<d ₂ /d ₃<7  (1)−0.04<f/f ₃<0.04  (2)where d₂ is the thickness of the first lens on the optical axis, d₃ is aspace distance between the first and second lenses on the optical axis,f is a focal length of the whole of the wide angle optical system, andf₃ is a focal length of the third lens.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (3):0.30<h _(m7) /h _(P7)<2.0  (3)where h_(m7) is the shortest distance between the optical axis and apoint on the image-side surface of the third lens through which a lightray passing through the center of the aperture stop at an angle of 36degrees to the optical axis to be used for an image formation passes,and h_(P7) is the shortest distance between the optical axis and themost image-side point on the image-side surface of the third lens.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (4):0.06<d _(P7) /f<0.3  (4)where d_(P7) is the distance between: a point at which the image-sidesurface of the third lens and the optical axis cross each other; and apoint on the optical axis at which the distance between the mostimage-side point on the image-side surface of the third lens and theoptical axis becomes the shortest distance.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (5):10<ν₂<25  (5)where ν₂ is the Abbe's number of the second lens.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (6):0.9<r ₂ /r ₃<4.4  (6)where r₂ is the radius of curvature of the image-side surface of thefirst lens, and r₃ is the radius of curvature of the object-side surfaceof the second lens.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (7):0.3<|Hr/Σd|<1.4  (7)where Hr is the distance from an image formation position to a positionof a back principal point of the whole of the wide angle optical system,and Σd is the total length of the whole of the wide angle opticalsystem.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (8):−2.0<Exp/f<−0.45  (8)where Exp is the distance from an image formation position to an exitpupil position.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (9):0.7<L3d/Fno _(Min)<0.98  (9)where L3 d is the distance, in millimeters (mm), from the object-sidesurface of the third lens to an image formation position, and Fno_(Min)is the minimum F-number.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (10):25%<ILL<45%  (10)where ILL is the ratio of quantity of light of off-axis light flux toquantity of light of on-axis light flux at an image formation position.

Also, a wide angle optical system according to the present inventionpreferably satisfies the following condition (11):0.07<(r ₁ +r ₂)/(r ₁ −r ₂)<0.8  (11)where r₁ is the radius of curvature of the object-side surface of thefirst lens, and r₂ is the radius of curvature of the image-side surfaceof the first lens.

An image pickup apparatus according to the present invention ischaracterized in that the image pickup apparatus comprises any one ofthe above-described wide angle optical systems and an image pickupelement which is arranged on the image side of the wide angle opticalsystem and transforms an optical image into electrical signals.

The present invention can offer: a wide angle optical system which makesit possible to image a wide area and it possible to even form a fullycorrected image of a close object; and image pickup apparatus using thesame.

These and other features and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an image pickup apparatus accordingto the present invention and showing parameters which are involved inthe conditions (3) and (4).

FIG. 2 is a sectional view showing the formation of an image pickupapparatus provided with a wide angle optical system according to thefirst embodiment, taken along the optical axis.

FIGS. 3A, 3B, 3C, and 3D are diagrams showing spherical aberration,astigmatism, distortion, and chromatic aberration of magnification ofthe wide angle optical system shown in FIG. 2, respectively.

FIG. 4 is a sectional view showing the formation of an image pickupapparatus provided with a wide angle optical system according to thesecond embodiment, taken along the optical axis.

FIGS. 5A, 5B, 5C, and 5D are diagrams showing spherical aberration,astigmatism, distortion, and chromatic aberration of magnification ofthe wide angle optical system shown in FIG. 4, respectively.

FIG. 6 is a sectional view showing the formation of an image pickupapparatus provided with a wide angle optical system according to thethird embodiment, taken along the optical axis.

FIGS. 7A, 7B, 7C, and 7D are diagrams showing spherical aberration,astigmatism, distortion, and chromatic aberration of magnification ofthe wide angle optical system shown in FIG. 6, respectively.

FIG. 8 is a sectional view showing the formation of an image pickupapparatus provided with a wide angle optical system according to thefourth embodiment, taken along the optical axis.

FIGS. 9A, 9B, 9C, and 9D are diagrams showing spherical aberration,astigmatism, distortion, and chromatic aberration of magnification ofthe wide angle optical system shown in FIG. 8, respectively.

FIG. 10 is a front perspective view showing the appearance of oneexample of a digital camera into which an image pickup apparatus using awide angle optical system of the present invention is incorporated.

FIG. 11 is a rear perspective view of the digital camera shown in FIG.10.

FIG. 12 is a front perspective view of one example of a personalcomputer the cover of which is opened and into which an image pickupapparatus using a wide angle optical system of the present invention isincorporated.

FIG. 13 is a side view of the personal computer shown in FIG. 12.

FIGS. 14A and 14B are a front view and a side view of one example of acellular phone into which an image pickup apparatus using a wide angleoptical system of the present invention is incorporated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before undertaking the description of the embodiments of a wide angleoptical system of the present invention and an image pickup apparatususing the same, the operation and effects by the formations of thepresent invention will be explained. In the present invention, a lensthe paraxial focal length of which has a positive value is treated as apositive lens, and a lens the paraxial focal length of which has anegative value is treated as a negative lens.

A wide angle optical system of the present invention comprises, in orderfrom the object side, an aperture stop, a first lens with positiverefracting power, a second lens with negative refracting power, and athird lens.

When a telephoto type optical system becomes wide angle, the incidentangle of an off-axis principal ray incident on the image pickup surfaceof an image pickup element arranged on the image side of the telephototype optical system will become larger. As a result, the sensitivity ofthe image pickup element deteriorates on the periphery of the imagepickup element. However, when an aperture stop is arranged at the objectside than every lens as in the wide angle optical system of the presentinvention, the exit pupil can be kept away from the image surface, sothat it is possible to decrease the angle of incidence of light raywhich is incident on the periphery of the image pickup element. As aresult, it is possible to avoid the deteriorating sensitivity on theperiphery of the image pickup element.

Also, the wide angle optical system according to the present inventionis formed so as to satisfy the following condition (1):2.2<d ₂ /d ₃<7  (1)where d₂ is the thickness of the first lens on the optical axis, and d₃is a space distance between the first and second lenses on the opticalaxis.

The condition (1) is used for prescribing the position of the image-sidesurface of the first lens. If d₂/d₃ is below the lower limit value ofthe condition (1), the image-side surface of the first lens becomes toonear to the object-side surface of the first lens, so that thedifference between the heights of light rays in the two surfaces of thefirst lens becomes small. As a result, it is impossible to correct anaberration well, and especially, it is impossible to correct a coma wellin an optical system having a wide range of image heights like a wideangle optical system according to the present invention. On the otherhand, if d₂/d₃ is beyond the upper limit value of the condition (1), theimage-side surface of the first lens becomes too distant from theobject-side surface of the first lens, so that it is impossible tocorrect well an axial aberration in consideration for aberrationsoccurring in the surfaces of the other lenses except the first lens. Inaddition, the image-side surface of the first lens becomes too near tothe object-side surface of the second lens in this case, so that it isimpossible to correct an aberration related to a principal ray such as afield curvature and an astigmatism, and an aberration such as a coma,with the corrections for the aberrations well-balanced.

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (1)′instead of the condition (1):2.6<d ₂ /d ₃<7  (1)′

Also, the wide angle optical system according to the present inventionis formed in such a way that: both surfaces of the third lens are anaspherical surface in which refracting power varies in accordance withdistance from the optical axis in such a way that the both surfaces havea convex shape facing toward the object side in the vicinity of theoptical axis and have a concave shape facing toward the object side inthe vicinity of the outer circumference of the lens; and the followingcondition (2) is satisfied:−0.04<f/f ₃<0.04  (2)where f is a focal length of the whole of the wide angle optical systemand f₃ is a focal length of the third lens.

The above-described shapes of both surfaces of the third lens and thecondition (2) satisfied by the wide angle optical system make itpossible to put the principal point at a position on the object side ofthe optical system without negating the effect of the second lens withnegative refracting power. As a result, it is possible to sufficientlyshorten the total length of the optical system to the focal length.

If f/f₃ is below the lower limit value of the condition (2), the thirdlens having a high dispersion characteristic has too large refractingpower in the central portion of the third lens, so that an axialchromatic aberration remarkably occurs and makes the image qualitydeteriorate. On the other hand, if f/f₃ is beyond the upper limit valueof the condition (2), the refracting power in the central portion of thethird lens becomes too small and the diverging action also becomes weak,so that the position of the principal point shifts to the image side. Asa result, the optical system becomes large.

If the wide angle optical system according to the present invention isformed so as to satisfy the following condition (2)′ instead of thecondition (2), the difference between on-axis refracting power andoff-axis refracting power of the third lens is easily made, so that itis easily possible to downsize the optical system more, and a distortionis easy to correct:−0.035<f/f ₃<0.025  (2)′

In this case, the upper limit value of the condition (2)′ may bereplaced with the upper limit value of the condition (2), or the lowerlimit value of the condition (2)′ may be replaced with the lower limitvalue of the condition (2).

Also, it is preferred that, in the wide angle optical system of thepresent invention, the parameters shown in FIG. 1 satisfy the followingcondition (3):0.30<h _(m7) /h _(P7)<2.0  (3)where h_(m7) is the shortest distance between the optical axis and apoint on the image-side surface of the third lens through which a lightray passing through the center of the aperture stop S at an angle of 36degrees to the optical axis to be used for an image formation passes,and h_(P7) is the shortest distance between the optical axis and themost image-side point on the image-side surface of the third lens.

FIG. 1 is a schematic view showing an image pickup apparatus whichcomprises a wide angle optical system according to the presentinvention, a cover glass, and an image pickup element such as a CCD(Charge Coupled Device) and a CMOS (Complementary Metal-OxideSemiconductor). In addition, FIG. 1 also shows the parameters which areinvolved in the conditions (3) and (4) respectively.

In FIG. 1, S is the aperture stop, L₁ is the first lens, L₂ is thesecond lens, L₃ is the third lens, CG is the cover glass, IM is theimage pickup surface of the image pickup element, L_(c) is the opticalaxis, L_(m) is the light ray which passes through the center of theaperture stop S at an angle of 36 degrees to the optical axis L_(c) toform an image on the image pickup surface IM, P_(C7) is the point atwhich the image-side surface of the third lens L₃ and the optical axisL_(c) cross each other, P_(m7) is the point on the image-side surface ofthe third lens L₃ through which the light ray L_(m) passes, P_(P7) isthe most image-side point on the image-side surface of the third lensL₃, h_(m7) is the shortest distance from the point P_(m7) to the opticalaxis L_(c), h_(P7) is the shortest distance from the point P_(P7) to theoptical axis L_(c), and d_(P7) is the distance between: the point on theoptical axis L_(c) at which the distance from the point P_(P7) to theoptical axis L_(c) becomes the shortest distance; and the point P_(C7).

When the wide angle optical system according to the present invention inwhich both surfaces of the third lens have a concave shape facing towardthe object side in the vicinity of the outer circumference of the lensformed in such a way that the wide angle optical system satisfies thecondition (3), such formation of the wide angle optical systemsatisfying the condition (3) can make a small angle of incidence of anoff-axis principal ray which is incident on the periphery of the imagepickup element in the case of an arrangement of the image pickup elementon the image side of the optical system. As a result, it is possible toavoid the deteriorating sensitivity of the image pickup element on theperiphery of the image pickup element. In addition, such formation alsomakes it possible to correct a pincushion distortion occurring with thesecond lens.

If h_(m7)/h_(P7) is below the lower limit value of the condition (3),the third lens having a high dispersion characteristic has too largerefracting power in the central portion of the third lens, so that anaxial chromatic aberration remarkably occurs and makes the image qualitydeteriorate. On the other hand, if h_(m7)/h_(P7) is beyond the upperlimit value of the condition (3), the convergence action of the thirdlens in the vicinity of the outer circumference of the third lensbecomes too weak, so that the weak convergence action causes a largeangle of incidence of an off-axis principal ray which is incident on theperiphery of the image pickup element. As a result, it is impossible tosufficiently avoid the deteriorating sensitivity of the image pickupelement on the periphery of the image pickup element.

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (3)′instead of the condition (3):0.50<h _(m7) /h _(P7)<1.24  (3)′In this case, the upper limit value of the condition (3)′ may bereplaced with the upper limit value of the condition (3), or the lowerlimit value of the condition (3)′ may be replaced with the lower limitvalue of the condition (3).

Also, it is preferred that, in the wide angle optical system of thepresent invention, the parameters shown in FIG. 1 satisfy the followingcondition (4):0.06<d _(P7) /f<0.3  (4)where d_(P7) is the distance between: a point at which the image-sidesurface of the third lens and the optical axis cross each other; and apoint on the optical axis at which the distance between the mostimage-side point on the image-side surface of the third lens and theoptical axis becomes the shortest distance.

When the wide angle optical system according to the present invention,in which both surfaces of the third lens are shaped to be convex towardthe object side in the vicinity of the optical axis and to be concavetoward the object side in the vicinity of the outer circumference of thelens, is configured to satisfy the condition (4), an image pickupelement, if arranged on the image side of the optical system, canreceive, on the periphery thereof, an off-axis principal ray incident ata small angle of incidence. As a result, it is possible to avoiddeteriorating sensitivity on the periphery of the image pickup element.In addition, such a configuration also makes it possible to correct adistortion.

If d_(P7)/f is below the lower limit value of the condition (4), thethird lens with the high dispersion characteristic has so large arefracting power in the central portion of the third lens that an axialchromatic aberration is conspicuously generated to degrade the imagequality. Furthermore, the convergence action of the third lens in thevicinity of the outer circumference becomes so weak as to give a largeangle of incidence to an off-axis principal ray incident on theperiphery of the image pickup element. As a result, it is impossible tosufficiently avoid deteriorating sensitivity on the periphery of theimage pickup element. On the other hand, if d_(P7)/f is above the upperlimit of the condition (4), On the other hand, if d_(P7)/f is above theupper limit of the condition (4), a difference between the lens actionon on-axis light rays and the lens action on off-axis light rays at theimage-side surface of the third lens is rendered too small toeffectively correct a distortion.

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (4)′instead of the condition (4):0.09<d _(P7) /f<0.19  (4)′In this case, the upper limit value of the condition (4)′ may bereplaced with the upper limit value of the condition (4), or the lowerlimit value of the condition (4)′ may be replaced with the lower limitvalue of the condition (4).

Also, it is preferred that the wide angle optical system of the presentinvention satisfies the following condition (5):10<ν₂<25  (5)where ν₂ is the Abbe's number of the second lens.

In order to give a wide angle capability to an optical system like awide angle optical system according to the present invention, theoptical system must be formed in such a way that the focal length of thefirst lens is small. However, the small focal length of the first lenscauses the occurrence of a large chromatic aberration. In a wide angleoptical system according to the present invention, both surfaces of thethird lens are an aspherical surface in which refracting power varies inaccordance with distance from the optical axis in such a way that theboth surfaces have a convex shape facing toward the object side in thevicinity of the optical axis and have a concave shape facing toward theobject side in the vicinity of the outer circumference of the lens. Thatis to say, the image-side surface of the third lens is a surface havinginflection points, so that a correction of an axial chromatic aberrationin the third lens causes a large chromatic aberration of magnificationoccurring in the vicinity of the outer circumference of the third lens.Accordingly, a wide angle optical system according to the presentinvention is preferably formed in such a way that the second lens ismade of a material having a high dispersion characteristic in order tocorrect a chromatic aberration of magnification. In addition, the secondlens is preferably given suitable refracting power in order to prevent acoma from increasing. The condition (5) is used for prescribing suchsecond lens.

If ν₂ is below the lower limit value of the condition (5), it is hard toobtain a material which has good optical characteristics, especiallygood optical characteristics for transmittance and scattering of lighton the short-wavelength side. On the other hand, if ν₂ is beyond theupper limit value of the condition (5), it is hard to remove a chromaticaberration of magnification in the second lens when the wide angleoptical system according to the present invention satisfies thecondition (2), so that it is hard to improve the ability of the thirdlens in the vicinity of the outer circumference of the third lens.

Also, it is preferred that the wide angle optical system of the presentinvention satisfies the following condition (6):0.9<r ₂ /r ₃<4.4  (6)where r₂ is the radius of curvature of the image-side surface of thefirst lens, and r₃ is the radius of curvature of the object-side surfaceof the second lens.

The condition (6) is used for prescribing the condition for making asmall air space between the first and second lenses without aggravatingan aberration. The formation of a wide angle optical system according tothe present invention satisfying the condition (6) makes it possible toarrange the first and second lenses with the first lens close to thesecond lens, so that the whole of the wide angle optical system is easyto downsize. In addition, because the formation satisfying the condition(6) makes it possible to make a small air space between the first andsecond lenses, the condition (6) makes it possible to ensure a distancefrom the object-side surface of the second lens to the image-sidesurface of the third lens, so that the wide angle optical system easilysatisfies the condition (1).

If r₂/r₃ is below the lower limit value of the condition (6), coma oftenoccurs. On the other hand, if r₂/r₃ is beyond the upper limit value ofthe condition (6), the air space between the first and second lensesmust be large, so that the whole of the wide angle optical system ishard to downsize and it is hard for the wide angle optical system tosatisfy the condition (1).

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (6)′instead of the condition (6):1.2<r ₂ /r ₃<2.8  (6)′In this case, the upper limit value of the condition (6)′ may bereplaced with the upper limit value of the condition (6), or the lowerlimit value of the condition (6)′ may be replaced with the lower limitvalue of the condition (6).

Also, it is preferred that the wide angle optical system of the presentinvention satisfies the following condition (7):0.3<|Hr/Σd|<1.4  (7)where Hr is the distance from an image formation position to a positionof a back principal point of the whole of the wide angle optical system,and Σd is the total length of the whole of the wide angle opticalsystem.

The condition (7) is used for prescribing a condition for making a smalltotal length of the wide angle optical system while holding occurrenceof the field curvature in check. If |Hr/Σd| is below the lower limitvalue of the condition (7), the total length of the wide angle opticalsystem becomes large. On the other hand, if |Hr/Σd| is beyond the upperlimit value of the condition (7), the angle of view to the total lengthof the wide angle optical system becomes too large. As a result, it ishard to hold occurrence of the field curvature in check.

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (7)′instead of the condition (7):0.46<|Hr/Σd|<0.89  (7)′In this case, the upper limit value of the condition (7)′ may bereplaced with the upper limit value of the condition (7), or the lowerlimit value of the condition (7)′ may be replaced with the lower limitvalue of the condition (7).

Also, it is preferred that the wide angle optical system of the presentinvention satisfies the following condition (8):−2.0<Exp/f<−0.45  (8)where Exp is the distance from an image formation position to an exitpupil position.

The condition (8) is used for prescribing a condition for makingdownsizing of the wide angle optical system compatible with preservationof the peripheral performance. If Exp/f is below the lower limit valueof the condition (8), the wide angle optical system requires the exitpupil far from an image formation position, so that the wide angleoptical system becomes large. On the other hand, if Exp/f is beyond theupper limit value of the condition (8), the angle of incidence of alight ray which is incident on the periphery of the image pickup elementbecomes too large in the case of the arrangement of the image pickupelement on the image side of the optical system, so that the peripheralperformance remarkably deteriorates.

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (8)′instead of the condition (8):−1.3<Exp/f<−0.58  (8)′In this case, the upper limit value of the condition (8)′ may bereplaced with the upper limit value of the condition (8), or the lowerlimit value of the condition (8)′ may be replaced with the lower limitvalue of the condition (8).

Also, it is preferred that the wide angle optical system of the presentinvention satisfies the following condition (9):0.7<L3d/Fno _(Min)<0.98  (9)where L3 d is the distance, in millimeters (mm), from the object-sidesurface of the third lens to an image formation position, and Fno_(Min)is the minimum F-number.

If L3 d/Fno_(Min) is below the lower limit value of the condition (9),the F-number becomes large or the object-side surface of the third lensbecomes close to the image formation position, so that foreign objectsin the optical system stand out. On the other hand, if L3 d/Fno_(Min) isbeyond the upper limit value of the condition (9), the distance from theobject-side surface of the third lens to the image formation positionbecomes long, so that the wide angle optical system becomes large.

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (9)′instead of the condition (9):0.79<L3d/Fno _(Min)<0.98  (9)′

Also, it is preferred that the wide angle optical system of the presentinvention satisfies the following condition (10):25%<ILL<45%  (10)where ILL is the ratio of quantity of light of off-axis light flux toquantity of light of on-axis light flux at an image formation position.

When an optical system is formed like a wide angle optical systemaccording to the present invention in such a way that the optical systemcomprises, in order from the object side, positive refracting power andnegative refracting power, a positive distortion occurs in the opticalsystem. If an image pickup element arranged on the image side of suchoptical system is displaced along the direction perpendicular to theoptical axis so that the image pickup element is not aligned with a lensframe on the optical axis, then a distortion will occur asymmetrically.As a result, quantity of light in the peripheral part becomes asymmetry,so that the asymmetric quantity of light in the peripheral part causesdeteriorated image quality. However, the formation of the wide angleoptical system satisfying the condition (10) can prevent occurrence ofan asymmetric distortion, so that quantity of light in the central partis well-balanced with quantity of light in the peripheral part. As aresult, it is possible to prevent image quality from deteriorating.

If ILL is below the lower limit value of the condition (10), the balanceof the quantity of light in the peripheral part is lost, so that it isdifficult to brighten an image by adjusting the brightness of the imageby an electrical correction and asymmetry of the quantity of light inthe peripheral part also easily stands out. On the other hand, if ILL isbeyond the upper limit value of the condition (10), although the balanceof the quantity of light in the peripheral part is improved, a balancebetween refracting powers of the lenses is lost in the wide angleoptical system which is given a wide angle capability. In particular, abalance between refracting powers of the first and second lenses islost. As a result, the formation of ILL beyond the upper limit valueleads to: coma, field curvature, and astigmatism which easily occur; orthe large total length of the optical system.

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (10)′instead of the condition (10):25%<ILL<42%  (10)′

Also, it is preferred that the wide angle optical system of the presentinvention satisfies the following condition (11):0.07<(r ₁ +r ₂)/(r ₁ −r ₂)<0.8  (11)where r₁ is the radius of curvature of the object-side surface of thefirst lens, and r₂ is the radius of curvature of the image-side surfaceof the first lens.

The first lens of the wide angle optical system according to the presentinvention has the smallest focal length in the wide angle opticalsystem, so that the first lens is most susceptible to an assembly errorin making the wide angle optical system. However, the formation of thewide angle optical system satisfying the condition (11) makes itpossible to restrain the influence of variations in the quality ofproducts in manufacturing and gives good optical characteristics to thewide angle optical system.

If (r₁+r₂)/(r₁−r₂) is below the lower limit value of the condition (11),the radius of curvature of the object-side surface of the first lensbecomes too small, so that the inclination of light incident on theobject-side surface of the first lens becomes large to the normal of theobject-side surface of the first lens. As a result, the largeinclination of light causes an increase of sensitivity of the lenses todecentration. On the other hand, if (r₁+r₂)/(r₁−r₂) is beyond the upperlimit value of the condition (11), the radius of curvature of theimage-side surface of the first lens becomes too large, so that theinclination of light exiting from the image-side surface of the firstlens becomes large to the normal of the image-side surface of the firstlens. As a result, the large inclination of light causes an increase ofsensitivity of the lenses to decentration.

It is more preferable if the wide angle optical system of the presentinvention is formed so as to satisfy the following condition (11)′instead of the condition (11):0.09<(r ₁ +r ₂)/(r ₁ −r ₂)<0.49  (11)′In this case, the upper limit value of the condition (11)′ may bereplaced with the upper limit value of the condition (11), or the lowerlimit value of the condition (11)′ may be replaced with the lower limitvalue of the condition (11).

Also, it is preferred that all lenses used for the wide angle opticalsystem are made of resin in the wide angle optical system of the presentinvention.

Also, it is preferred that a shutter is arranged on the object side ofthe aperture stop in the wide angle optical system of the presentinvention. The shutter may be integrated with the aperture stop or bemade as a different part from the aperture stop.

Besides, in the wide angle optical system of the present invention, theaperture stop is preferably a variable stop.

The first to fourth embodiments according to the present invention willbe explained below referring to the drawings.

In the drawings, subscript numerals in r₁, r₂, . . . and d₁, d₂, . . .in sectional views of the optical system correspond to surface numbers,1, 2, . . . in numerical data, respectively. Further, in views showingaberration curves, ΔM in views for astigmatism denotes astigmatism in ameridional plane, and ΔS in views for astigmatism denotes astigmatism ina sagittal plane. In this case, the meridional plane is a plane (planeparallel to this document plane) including the optical axis and thechief ray of an optical system. The sagittal plane is a plane (planeperpendicular to this document plane) perpendicular to a plane includingthe optical axis and the chief ray of an optical system. In addition,FIY denotes an image height.

Further, in the numerical data of the lens in each of the followingembodiments, s denotes a surface number of the lens, r denotes theradius of curvature of each surface, d denotes a surface interval, nddenotes the refractive index at d line (which has a wave length of587.5600 nm), νd denotes the Abbe's number to the d line, K denotes aconical coefficient, and A₄, A₆, A₈, and A₁₀ denote aspherical surfacecoefficients, respectively.

In the data for the aspherical surface coefficients in the followingnumerical data, E denotes a power of ten. For example, “E-01” denotes“ten to the power of minus one”. In addition, the shape of eachaspherical surface is expressed by the following equation withaspherical surface coefficients in each embodiment:Z=(Y ² /r)/[1+{1−(1+K)(Y/r)²}^(1/2) ]+A ₄ Y ⁴ +A ₆ Y ⁶ +A ₈ Y ⁸ +A ₁₀ Y¹⁰+ . . .where, Z is taken as a coordinate in the direction along the opticalaxis, and Y is taken as a coordinate in the direction perpendicular tothe optical axis.

Embodiment 1

FIG. 2 is a sectional view showing the formation of an image pickupapparatus provided with a wide angle optical system according to thepresent embodiment, taken along the optical axis. FIGS. 3A, 3B, 3C, and3D are views showing a spherical aberration, an astigmatism, adistortion, and a chromatic aberration of magnification respectively, inthe wide angle optical system shown in FIG. 2.

First, the formation of an image pickup apparatus provided with a wideangle optical system of the present embodiment is explained using FIG.2. In the image pick up apparatus provided with the wide angle opticalsystem of the present embodiment, in order from the object side, anaperture stop S, a first lens L₁, a second lens L₂, a third lens L₃, aCCD cover glass CG, and a CCD having an image pickup surface IM arearranged on the optical axis L_(c). Further, a low pass filter which isgiven an IR-cut coating, or the like may be arranged between the thirdlens L₃ and the CCD cover glass CG.

The first lens L₁ is a biconvex lens both surfaces of which are anaspherical surface. The second lens L₂ is a negative meniscus lens bothsurfaces of which are an aspherical surface, and the convex shape ofeach of both surfaces of the second lens L₂ faces toward the image sideof the wide angle optical system. The third lens L₃ is a meniscus lensboth surfaces of which are an aspherical surface in which refractingpower varies in accordance with distance from the optical axis in such away that the both surfaces have a convex shape facing toward the objectside in the vicinity of the optical axis and have a concave shape facingtoward the object side in the vicinity of the outer circumference.

Next, the constitution and numerical data of lenses which constituteeach optical system according to the present embodiment are shown.

Numerical value data 1 Unit: millimeter (mm) Surface data effective s rd nd νd diameter 1 (aperture stop) ∞ 0.20 0.65 2 (aspherical surface)2.528 1.18 1.52559 56.45 0.90 3 (aspherical surface) −1.254 0.20 1.09 4(aspherical surface) −0.739 1.02 1.58393 30.21 1.98 5 (asphericalsurface) −1.691 0.63 1.39 6 (aspherical surface) 1.612 0.61 1.5839330.21 2.06 7 (aspherical surface) 1.351 0.86 2.36 8 ∞ 0.50 1.51633 64.142.60 9 ∞ 0.30 2.71 image plane ∞ Aspherical surface data The secondsurface K = −5.621, A4 = 1.40251E−02, A6 = −5.20761E−02 The thirdsurface K = −0.965, A4 = −4.01327E−02 The fourth surface K = −0.739, A4= 1.76074E−01, A6 = 1.51755E−02 The fifth surface K = 0.067, A4 =9.18177E−02, A6 = 2.68669E−02 The sixth surface K = −2.207, A4 =−4.43963E−02, A6 = 1.25664E−03 The seventh surface K = −2.929, A4 =−2.72328E−02, A6 = −7.55043E−04 Various data Focal length 3.64 F-number2.8 Angle of view −37.6° Image height 2.8 The total length of lens 5.3Back focus 1.49 Data regarding the above condition in the firstembodiment Condition (1) (2.2 < d₂/d₃ < 7) 5.90 Condition (2) (−0.04 <f/f₃ < 0.04) −0.0353 Condition (3) (0.30 < h_(m7)/h_(P7) < 2.0) 0.943Condition (4) (0.06 < d_(p7)/f < 0.3) 0.128 Condition (5) (10 < ν₂ < 25)30.21 Condition (6) (0.9 < r₂/r₃ < 4.4) 1.69 Condition (7) (0.3 <|Hr/Σd| < 1.4) 0.67 Condition (8) (−2.0 < Exp/f < −0.45) −1.15 Condition(9) (0.7 < L3d/Fno_(Min) ≦ 0.98) 0.81 Condition(10) (25% < ILL < 45%)31.8 Condition(11) (0.07 < (r₁ + r₂)/(r₁ − r₂) < 0.8) 0.34

Embodiment 2

FIG. 4 is a sectional view showing the formation of an image pickupapparatus provided with a wide angle optical system according to thepresent embodiment, taken along the optical axis. FIGS. 5A, 5B, 5C, and5D are views showing a spherical aberration, an astigmatism, adistortion, and a chromatic aberration of magnification respectively, inthe wide angle optical system shown in FIG. 4.

First, the formation of an image pickup apparatus provided with a wideangle optical system of the present embodiment is explained using FIG.4. In the image pickup apparatus provided with the wide angle opticalsystem of the present embodiment, in order from the object side, anaperture stop S, a first lens L₁, a second lens L₂, a third lens L₃, aCCD cover glass CG, and a CCD having an image pickup surface IM arearranged on the optical axis L_(c). Further, a low pass filter which isgiven an IR-cut coating, or the like may be arranged between the thirdlens L₃ and the CCD cover glass CG.

The first lens L₁ is a biconvex lens both surfaces of which are anaspherical surface. The second lens L₂ is a negative meniscus lens bothsurfaces of which are an aspherical surface, and the convex shape ofeach of both surfaces of the second lens L₂ faces toward the image sideof the wide angle optical system. The third lens L₃ is a meniscus lensboth surfaces of which are an aspherical surface in which refractingpower varies in accordance with distance from the optical axis in such away that the both surfaces have a convex shape facing toward the objectside in the vicinity of the optical axis and have a concave shape facingtoward the object side in the vicinity of the outer circumference.

Next, the constitution and numerical data of lenses which constituteeach optical system according to the present embodiment are shown.

Numerical value data 2 Unit: millimeter (mm) Surface data effective s rd nd νd diameter 1 (aperture stop) ∞ 0.20 0.67 2 (aspherical surface)3.315 1.15 1.53071 55.69 0.86 3 (aspherical surface) −1.512 0.42 1.12 4(aspherical surface) −0.760 0.88 1.58393 30.21 1.15 5 (asphericalsurface) −1.466 0.50 1.38 6 (aspherical surface) 2.626 1.11 1.5307155.69 1.94 7 (aspherical surface) 2.216 0.79 2.43 8 ∞ 0.30 1.51633 64.142.64 9 ∞ 0.30 2.70 image plane ∞ Aspherical surface data The secondsurface K = −1.840, A4 = −4.59100E−02, A6 = −3.0510E−02, A8 =−7.12400E−02 The third surface K = −1.218, A4 = −7.36900E−02, A6 =−2.97600E−02, A8 = 3.09400E−02, A10 = −2.59300E−02 The fourth surface K= −0.926, A4 = 1.49100E−01, A6 = 1.07400E−01, A8 = −8.17500E−02, A10 =2.31100E−02 The fifth surface K = −0.490, A4 = 7.28800E−02, A6 =5.67100E−02, A8 = −1.36400E−02, A10 = 8.56100E−04 The sixth surface K =−7.093, A4 = −2.29700E−02, A6 = −6.43000E−04, A8 = 1.18900E−03, A10 =−2.30700E−04 The seventh surface K = −1.369, A4 = −6.13500E−02, A6 =9.18900E−03, A8 = −7.47900E−04, A10 = −2.48600E−06 Various data Focallength 3.73 F-number 2.8 Angle of view −36.8° Image height 2.8 The totallength of lens 5.5 Back focus 1.28 Data regarding the above condition inthe second embodiment Condition (1) (2.2 < d₂/d₃ < 7) 2.74 Condition (2)(−0.04 < f/f₃ < 0.04) −0.0084 Condition (3) (0.30 < h_(m7)/h_(P7) < 2.0)0.937 Condition (4) (0.06 < d_(P7)/f < 0.3) 0.074 Condition (5) (10 < ν₂< 25) 30.21 Condition (6) (0.9 < r₂/r₃ < 4.4) 1.99 Condition (7) (0.3 <|Hr/Σd| < 1.4) 0.66 Condition (8) (−2.0 < Exp/f < −0.45) −1.15 Condition(9) (0.7 < L3d/Fno_(Min) ≦ 0.98) 0.89 Condition(10) (25% < ILL < 45%)39.9 Condition(11) (0.07 < (r₁ + r₂)/(r₁ − r₂) < 0.8) 0.37

Embodiment 3

FIG. 6 is a sectional view showing the formation of an image pickupapparatus provided with a wide angle optical system according to thepresent embodiment, taken along the optical axis. FIGS. 7A, 7B, 7C, and7D are views showing a spherical aberration, an astigmatism, adistortion, and a chromatic aberration of magnification respectively, inthe wide angle optical system shown in FIG. 6.

First, the formation of an image pickup apparatus provided with a wideangle optical system of the present embodiment is explained using FIG.6. In the image pickup apparatus provided with the wide angle opticalsystem of the present embodiment, in order from the object side, anaperture stop S, a first lens L₁, a second lens L₂, a third lens L₃, aCCD cover glass CG, and a CCD having an image pickup surface IM arearranged on the optical axis L_(c). Further, a low pass filter which isgiven an IR-cut coating, or the like may be arranged between the thirdlens L₃ and the CCD cover glass CG.

The first lens L₁ is a biconvex lens both surfaces of which are anaspherical surface. The second lens L₂ is a negative meniscus lens bothsurfaces of which are an aspherical surface, and the convex shape ofeach of the both surfaces of the second lens L₂ faces toward the imageside of the wide angle optical system. The third lens L₃ is a meniscuslens both surfaces of which are an aspherical surface in whichrefracting power varies in accordance with distance from the opticalaxis in such a way that the both surfaces have a convex shape facingtoward the object side in the vicinity of the optical axis and have aconcave shape facing toward the object side in the vicinity of the outercircumference.

Next, the constitution and numerical data of lenses which constituteeach optical system according to the present embodiment are shown.

Numerical value data 3 Unit: millimeter (mm) Surface data effective s rd nd νd diameter 1 (aperture stop) ∞ 0.10 0.68 2 (aspherical surface)3.076 1.11 1.53071 55.69 0.80 3 (aspherical surface) −1.651 0.42 1.05 4(aspherical surface) −0.838 0.80 1.63259 23.27 1.10 5 (asphericalsurface) −1.568 0.40 1.30 6 (aspherical surface) 3.271 1.39 1.5307155.69 1.67 7 (aspherical surface) 2.771 0.74 2.38 8 ∞ 0.30 1.51633 64.142.66 9 ∞ 0.30 2.72 image pickup surface ∞ Aspherical surface data Thesecond surface K = −1.561, A4 = −4.45188E−02, A6 = −2.78685E−02, A8 =−4.80497E−02 The third surface K = −1.110, A4 = −7.35754E−02, A6 =−2.18139E−03, A8 = 8.64574E−03, A10 = −1.50468E−02 The fourth surface K= −0.906, A4 = 1.40870E−01, A6 = 1.10717E−01, A8 = −7.36659E−02, A10 =1.76769E−02 The fifth surface K = −0.424, A4 = 6.85954E−02, A6 =5.20628E−02, A8 = −1.17039E−02, A10 = 1.12392E−03 The sixth surface K =−15.009, A4 = −1.96223E−02, A6 = −7.60260E−03, A8 = 3.02406E−03, A10 =−7.56075E−04 The seventh surface K = −2.040, A4 = −4.50247E−02, A6 =6.82918E−03, A8 = −1.05009E−03, A10 = 3.98509E−05 Various data Focallength 3.77 F-number 2.8 Angle of view −36.8° Image height 2.8 The totallength of lens 5.5 Back focus 1.23 Data regarding the above condition inthe third embodiment Condition (1) (2.2 < d₂/d₃ < 7) 2.64 Condition (2)(−0.04 < f/f₃ < 0.04) −0.004 Condition (3) (0.30 < h_(m7)/h_(P7) < 2.0)0.999 Condition (4) (0.06 < d_(P7)/f < 0.3) 0.055 Condition (5) (10 < ν₂< 25) 23.27 Condition (6) (0.9 < r₂/r₃ < 4.4) 1.96 Condition (7) (0.3 <|Hr/Σd| < 1.4) 0.67 Condition (8) (−2.0 < Exp/f < −0.45) −1.06 Condition(9) (0.7 < L3d/Fno_(Min) ≦ 0.98) 0.98 Condition(10) (25% < ILL < 45%)36.2 Condition(11) (0.07 < (r₁ + r₂)/(r₁ − r₂) < 0.8) 0.3

Embodiment 4

FIG. 8 is a sectional view showing the formation of an image pickupapparatus provided with a wide angle optical system according to thepresent embodiment, taken along the optical axis. FIGS. 9A, 9B, 9C, and9D are views showing a spherical aberration, an astigmatism, adistortion, and a chromatic aberration of magnification respectively, inthe wide angle optical system shown in FIG. 8.

First, the formation of an image pickup apparatus provided with a wideangle optical system of the present embodiment is explained using FIG.8. In the image pickup apparatus provided with the wide angle opticalsystem of the present embodiment, in order from the object side, anaperture stop S, a first lens L₁, a second lens L₂, a third lens L₃, aCCD cover glass CG, and a CCD having an image pickup surface IM arearranged on the optical axis L_(c). Further, a low pass filter which isgiven an IR-cut coating, or the like may be arranged between the thirdlens L₃ and the CCD cover glass CG.

The first lens L₁ is a biconvex lens both surfaces of which are anaspherical surface. The second lens L₂ is a negative meniscus lens bothsurfaces of which are an aspherical surface, and the convex shape ofeach of the both surfaces of the second lens L₂ faces toward the imageside of the wide angle optical system. The third lens L₃ is a meniscuslens both surfaces of which are an aspherical surface in whichrefracting power varies in accordance with distance from the opticalaxis in such a way that the both surfaces have a convex shape facingtoward the object side in the vicinity of the optical axis and have aconcave shape facing toward the object side in the vicinity of the outercircumference.

Next, the constitution and numerical data of lenses which constituteeach optical system according to the present embodiment are shown.

Numerical value data 4 Unit: millimeter (mm) Surface data effective s rd nd νd diameter 1 (aperture stop) ∞ 0.24 0.62 2 (aspherical surface)2.323 0.94 1.52559 56.45 0.92 3 (aspherical surface) −1.684 0.25 1.09 4(aspherical surface) −0.771 0.48 1.58393 30.21 1.19 5 (asphericalsurface) −1.418 0.71 1.18 6 (aspherical surface) 1.647 0.66 1.5255956.45 1.93 7 (aspherical surface) 1.420 0.50 2.25 8 ∞ 0.50 1.51633 64.142.47 9 ∞ 0.54 2.61 image plane ∞ Aspherical surface data The secondsurface K = 1.373, A4 = −6.48922E−02, A6 = −7.14622E−02, A8 =−3.64287E−02, A10 = 6.17409E−02 The third surface K = 0.000, A4 =−4.68358E−02, A6 = 2.92909E−02, A8 = 2.90073E−02, A10 = −8.75951E−03 Thefourth surface K = −1.002, A4 = 4.06561E−01, A6 = −1.21298E−02, A8 =−2.79473E−02, A10 = 3.67448E−03 The fifth surface K = −0.959, A4 =2.53800E−01, A6 = 6.85022E−02, A8 = −5.74446E−02, A10 = 1.15390E−02 Thesixth surface K = −5.072, A4 = −2.83124E−02, A6 = −1.16295E−02, A8 =3.75786E−03, A10 = −3.02246E−04 The seventh surface K = −4.453, A4 =−2.25374E−02, A6 = −7.90523E−03, A8 = 2.04710E−03, A10 = −1.94628E−04Various data Focal length 3.47 F-number 2.8 Angle of view −39.0° Imageheight 2.8 The total length of lens 4.6 Back focus 1.34 Data regardingthe above condition in the fourth embodiment Condition (1) (2.2 < d₂/d₃< 7) 3.76 Condition (2) (−0.04 < f/f₃ < 0.04) 0.0004 Condition (3) (0.30< h_(m7)/h_(P7) < 2.0) 0.995 Condition (4) (0.06 < d_(P7)/f < 0.3) 0.078Condition (5) (10 < ν₂ < 25) 30.21 Condition (6) (0.9 < r₂/r₃ < 4.4)2.18 Condition (7) (0.3 < |Hr/Σd| < 1.4) 0.74 Condition (8) (−2.0 <Exp/f < −0.45) −1.01 Condition (9) (0.7 < L3d/Fno_(Min) ≦ 0.98) 0.79Condition(10) (25% < ILL < 45%) 34.3 Condition(11) (0.07 < (r₁ + r₂)/(r₁− r₂) < 0.8) 0.16

Now, the above-described image pickup apparatuses using a wide angleoptical systems according to the present invention can be favorablyincorporated into a digital camera, a personal computer, or a cellularphone. Embodiments of a digital camera, personal computer, and acellular phone using the image pickup apparatuses are illustratedhereafter.

First, one example of a digital camera into which an image pickupapparatus using a wide angle optical system according to the presentinvention is incorporated is shown. FIG. 10 is a front perspective viewshowing the appearance of a digital camera into which an image pickupapparatus using a wide angle optical system according to the presentinvention is incorporated. FIG. 11 is a perspective back view showingthe digital camera shown in FIG. 10.

As shown in FIGS. 10 and 11, a digital camera 1 is provided with aphotographing opening section 101, a finder opening section 102, and aflash-light emitting section 103 in front thereof. Further, on the upperface of the camera, a shutter button 104 is provided. Further, on theback face of the camera, a liquid crystal display monitor 105, aninformation input section 106, and a finder eyepiece section 107 areprovided. In addition, within the camera, an image pickup apparatususing a wide angle optical system according to the present invention, aprocessing means, and a recording means are provided.

The digital camera 1 having such formation is formed in such a way thatimage information is acquired through the image pickup apparatus bypressing the shutter button 104 which is provided on the top thereof.The acquired image information is recorded by the recording means whichis provided inside the digital camera 1. In addition, the imageinformation recorded by the recording means is taken out by theprocessing means, and the image information can be also displayed as anelectronic image on the liquid crystal display monitor 105 which isprovided on the rear of the camera.

Next, one example of a personal computer which is an informationprocessing apparatus into which an image pickup apparatus using a wideangle optical system according to the present invention is incorporatedis shown. FIG. 12 is a front perspective view showing a personalcomputer the cover of which is opened and into which an image pickupapparatus using a wide angle optical system according to the presentinvention is incorporated. FIG. 13 is a side view of the same.

As shown in FIGS. 12 and 13, a personal computer 2 is provided with akeyboard 201 for a user to input information from the outside, and aliquid crystal display monitor 202 by which the user observes an image.And, a photographing opening section 203 is formed on the side of theliquid crystal display monitor 202. In addition, the personal computeris provided with an image pickup apparatus using a wide angle opticalsystem according to the present invention for photographing an image ofthe user or its surroundings, a processing means, and a recording meansinside the personal computer.

Further, the image information acquired by the image pickup apparatus isrecorded by the recording means. The image information recorded by therecording means is taken out from the recording means by the processingmeans, and the image information can be displayed as an electronic imageon the liquid crystal display monitor 202. It is also possible todisplay the electronic image on a personal computer of a communicationpartner by the processing means through the Internet or a telephonecircuit.

In this example, the image pickup apparatus is arranged on the side ofthe liquid crystal display monitor 202. However, an arranged position ofthe image pickup apparatus is not limited to the above-describedposition, and the image pickup apparatus may be arranged at anyposition, for example, at a position except the position on the side ofthe liquid crystal display monitor 202, such as a position over or underthe liquid crystal display monitor 202, or at a position on theperiphery of the keyboard 201.

In this example, a transmissive liquid crystal display element is usedas the liquid crystal display monitor 202, where the transmissive liquidcrystal display element is illuminated from its back by a backlightwhich is not shown in the drawings. However, a CRT display may be usedas a display means in this example.

Next, one example of a cellular phone which is an information processingapparatus into which an image pickup apparatus using a wide angleoptical system according to the present invention is incorporated isshown. FIG. 14A is an elevation showing one example of a cellular phoneinto which an image pickup apparatus using a wide angle optical systemaccording to the present invention is incorporated, and FIG. 14B is aside view showing the cellular phone.

As shown in FIGS. 14A and 14B, a cellular phone 3 is provided with amicrophone section 301 by which a user's voice is inputted asinformation, a speaker section 302 which outputs a voice of a user'spartner, input keys 303 by which a user inputs information, a liquidcrystal display monitor 304 which displays information including aphotographic image or the telephone number of a user or his partner, andan antenna 305 by which a radio wave for communication is sent andreceived. And a photographing opening section 306 is formed on the sideof the speaker section 302. In addition, the cellular phone 3 isprovided with an image pickup apparatus using a wide angle opticalsystem according to the present invention for photographing an image ofa user or its surroundings, a processing means, and a recording meansinside the cellular phone. Further, a liquid crystal display element isused as the liquid crystal display monitor 304. In the drawings, thearranged position of each of the components is not limited to theillustrated position of each of the components, and each of thecomponents of the cellular phone 3 may be arranged suitably.

Further, the image information acquired by the image pickup apparatus isrecorded by the recording means. And, the image information recorded bythe recording means is taken out from the recording means by theprocessing means, and the image information is displayed as anelectronic image on the liquid crystal display monitor 304. In addition,the processing means has a signal-processing function by whichinformation of an image to be sent to a communication partner istransformed into transmittable signals.

The invention claimed is:
 1. A wide angle optical system comprising, in order from an object side: an aperture stop; a first lens with positive refracting power; a second lens with negative refracting power; and a third lens, wherein both of an object-side surface and an image-side surface of the third lens are aspherical surfaces each of which is shaped to be convex toward the object side in a vicinity of an optical axis and to be concave toward the object side in a vicinity of an outer circumference of the third lens so that refracting power varies with distance from the optical axis, and wherein the following conditions (1)′, (2) and (3) are satisfied: 2.6<d ₂ /d ₃<7  (1)′ −0.04<f/f ₃<0.04  (2) 0.30<h _(m7) /h _(P7)<2.0  (3) where d₂ is a thickness of the first lens on the optical axis, d₃ is a space distance between the first lens and the second lens on the optical axis, f is a focal length of the wide angle optical system as a whole, f₃ is a focal length of the third lens, h_(m7) is a shortest distance between the optical axis and a point on the image-side surface of the third lens through which a light ray passing through a center of the aperture stop at an angle of 36 degrees to the optical axis passes for image formation, and h_(P7) is a shortest distance between the optical axis and a most image-side point on the image-side surface of the third lens.
 2. The wide angle optical system according to claim 1, wherein the following condition (4) is satisfied: 0.06<d _(P7) /f<0.3  (4) where d_(P7) is a distance between: a point at which the image-side surface of the third lens and the optical axis cross each other; and a point on the optical axis at which a distance between a most image-side point on the image-side surface of the third lens and the optical axis is shortest.
 3. The wide angle optical system according to claim 1, wherein the following condition (5) is satisfied: 10<v ₂<25  (5) where v₂ is an Abbe's number of the second lens.
 4. The wide angle optical system according to claim 1, wherein the following condition (6) is satisfied: 0.9<r ₂ /r ₃<4.4  (6) where r₂ is a radius of curvature of an image-side surface of the first lens, and r₃ is a radius of curvature of an object-side surface of the second lens.
 5. The wide angle optical system according to claim 1, wherein the following condition (8) is satisfied: −2.0<Exp/f<−0.45  (8) where Exp is a distance from an image formation position to an exit pupil position.
 6. The wide angle optical system according to claim 1, wherein the following condition (9) is satisfied: 0.7<L3d/Fno _(Min)<0.98  (9) where L3 d is a distance, in millimeters (mm), from the object-side surface of the third lens to an image formation position, and Fno_(Min) is a minimum F-number.
 7. The wide angle optical system according to claim 1, wherein the following condition (10) is satisfied: 25%<ILL<45%  (10) where ILL is a ratio of quantity of light of off-axis light flux to quantity of light of on-axis light flux at an image formation position.
 8. The wide angle optical system according to claim 1, wherein the following condition (11) is satisfied: 0.07<(r ₁ +r ₂)/(r ₁ −r ₂)<0.8  (11) where r₁ is a radius of curvature of an object-side surface of the first lens, and r₂ is a radius of curvature of an image-side surface of the first lens.
 9. A wide angle optical system comprising, in order from an object side: an aperture stop; a first lens with positive refracting power; a second lens with negative refracting power; and a third lens, wherein both of an object-side surface and an image-side surface of the third lens are aspherical surfaces each of which is shaped to be convex toward the object side in a vicinity of an optical axis and to be concave toward the object side in a vicinity of an outer circumference of the third lens so that refracting power varies with distance from the optical axis, and wherein the following conditions (1)′, (2) and (7) are satisfied: 2.6<d ₂ /d ₃<7  (1)′ −0.04<f/f ₃<0.04  (2) 0.3<|Hr/Σd|<4.4  (7) where d₂ is a thickness of the first lens on the optical axis, d₃ is a space distance between the first lens and the second lens on the optical axis, f is a focal length of the wide angle optical system as a whole, f₃ is a focal length of the third lens, Hr is a distance from an image formation position to a back principal point of the wide angle optical system as a whole, and Σd is a total length of the wide angle optical system as a whole.
 10. A wide angle optical system comprising, in order from an object side: an aperture stop; a first lens with positive refracting power; a second lens with negative refracting power; and a third lens, wherein both of an object-side surface and an image-side surface of the third lens are aspherical surfaces each of which is shaped to be convex toward the object side in a vicinity of an optical axis and to be concave toward the object side in a vicinity of an outer circumference of the third lens so that refracting power varies with distance from the optical axis, and wherein the following conditions (1), (2) and (11) are satisfied: 2.2<d ₂ /d ₃<7  (1) −0.04<f/f ₃<0.04  (2) 0.07<(r ₁ +r ₂)/(r ₁ −r ₂)<0.8  (11) where d₂ is a thickness of the first lens on the optical axis, d₃ is a space distance between the first lens and the second lens on the optical axis, f is a focal length of the wide angle optical system as a whole, f₃ is a focal length of the third lens, r₁ is a radius of curvature of an object-side surface of the first lens, and r₂ is a radius of curvature of an image-side surface of the first lens.
 11. The wide angle optical system according to claim 10, wherein the following condition (4) is satisfied: 0.06<d _(p7)/f<0.3  (4) where d_(p7) is a distance between: a point at which the image-side surface of the third lens and the optical axis cross each other; and a point on the optical axis at which a distance between a most image-side point on the image-side surface of the third lens and the optical axis is shortest.
 12. The wide angle optical system according to claim 10, wherein the following condition (6) is satisfied: 0.9<r ₂ /r ₃<4.4  (6) where r₂ is a radius of curvature of an image-side surface of the first lens, and r₃ is a radius of curvature of an object-side surface of the second lens.
 13. The wide angle optical system according to claim 10, wherein the following condition (8) is satisfied: −2.0<Exp/f<−0.45  (8) where Exp is a distance from an image formation position to an exit pupil position.
 14. The wide angle optical system according to claim 10, wherein the following condition (9) is satisfied: 0.7<L3d/Fno _(Min)≦0.98   (9) wherein L3d is a distance, in millimeters (mm), from the object-side surface of the third lens to an image formation position, and Fno_(Min), is a minimum F-number.
 15. An image pickup apparatus, comprising: the wide angle optical system according to claim 10; and an image pickup element which is arranged on an image side of the wide angle optical system, for converting an optical image into electrical signals. 